Method and device to improve backlight uniformity

ABSTRACT

A hot spot filter for a light guide is created by taking an image of the light output pattern of an illuminated light guide. The hot spot filter may be a film, a layer, or an additional liquid crystal display dedicated to attenuating bright spots from the light guide. The hot spot filter may be incorporated into the image display by adjusting the grey scale of individual pixels to provide sufficient compensation.

This is a divisional application of U.S. patent application Ser. No.11/467,763 filed Aug. 28, 2006, which is continuation application ofU.S. patent application Ser. No. 10/940,988, filed on Sep. 15, 2004.

FIELD OF THE INVENTION

The present invention relates to the field of liquid crystal displaysand, particularly, to the field of backlighting for liquid crystaldisplays and/or keypads.

BACKGROUND OF THE INVENTION

Liquid crystal displays are often illuminated by a backlight. Whendeveloping an LCD or keypad lighting system, many challenges areencountered that limit the ability to provide a uniformly lit lightsource to the display or keypad. For example, constraints on the x, y,and z size of the backlight system require tradeoffs or compromises inachieving proper and sufficient optical performance. Especially in thosecases where there is little space in the x or y directions (the planarextensions of the light guide), the light guide providing light to thedisplay from a light source (e.g., light emitting diodes—LEDs) or alight source directly illuminating a display may illuminate areas of thedisplay (or keypad) more strongly than other areas. The more stronglylit areas are often referred to as “hot spots” or “bright spots.”Currently, the options to compensate for hot spots in LED illuminatedLCDs and keypads are 1) moving the LEDs farther away from the lightguide; 2) increasing the number of LEDs; or 3) creating a complex anddifficult to tool optical dispersion patterns on the light guide plasticsurface. The current methods have shortcomings. If x, y space iscritical, then option (1) is not possible. Option (2) may be costprohibitive. Option (3) in which creating a complex pattern is formed onthe light guide, using current techniques, may increase the price of thelight guide, extend the development time, and require difficult and/orexpensive tooling for manufacture. Thus, there is a need to provide aneffective and cost effective method for eliminating hot spots from anilluminated light guide in a space constrained arrangement.

SUMMARY OF THE INVENTION

This invention addresses the problem of making light to a keypad ordisplay uniform in a space constrained device or system. In the presentinvention, a simple light guide and hot spot filter are presented. Thehot spot filter may be a separate layer, a display device, or patternssimply applied through a mask to the light guide surface. In creatingthe hot spot filter, an image from the illuminated light guide iscaptured to show precisely how the uniformity varies in the x and ydirections. Then, the measured data is used to create a mask that isapplied above or directly to the surface of the side of the light guidefacing the display or keypad. This may be accomplished by creating afilm that locally filters the hot spot and non-uniform regions. Forexample, if there are two strongly lit areas at the top of a module, themask may be a transparent piece of material with grey printing placed onthe transparent piece to correspond to the hot spots of the light guide.The net effect would be a uniform light output from the top surface ofthe light guide module. Alternatively, the image content of the LCD maybe dynamically changed to cover up or compensate for hot spots. That is,in the case where there are multiple hot spots, if a white screen wererequested by applications software, a white screen with grey regionscorresponding to the hot spots would be formed to reduce hot spotbrightness and provide a uniform image.

Other aspects and features of the present invention will become apparentto those of ordinary skill in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of present invention will now be described by way of examplewith reference to attached figures, wherein:

FIG. 1 is a block diagram that illustrates pertinent components of awireless communications device that communicate within a wirelesscommunication network according to the present invention;

FIG. 2 is a more detailed diagram of a preferred wireless communicationsdevice of FIG. 1 according to the present invention;

FIG. 3 illustrates an embodiment of a backlit liquid crystal display ofthe present invention;

FIG. 4 illustrates an embodiment of a transparent sheet with filters tocorrect for the hot spots of the light guide;

FIG. 5 illustrates an embodiment of an image capture method forcapturing an image from which a filter is formed;

FIG. 6 illustrates an embodiment of a method for forming a hot spotfilter; and

FIG. 7 illustrates an embodiment of a method for forming a hot spotcompensation map for light sources of different colours.

DETAILED DESCRIPTION

The present invention relates to a method and apparatus, especially amobile station such as a handheld communications device, that eliminatesbright spots (or hot spots) in the light output pattern from a lightguide that illuminates a display. Preferably, the light guide isilluminated by a light source that includes one or more light emittingdiodes (LEDs). The LEDs of the light source preferably will include red,green, and blue colours. Other colour schemes, such as cyan, magenta,and yellow, are contemplated by the present invention. Although thepresent invention is directed to a liquid crystal display per se, thepreferred use of the LCD is in a mobile station, such as a wirelessportable handheld communications device. Cell phones and pagers areamongst the many handheld devices contemplated by the present invention.Aside from illuminating a display, the method may be used to illuminatea keypad or keyboard, such as a keypad found on a mobile station, orother illuminable device or layer.

FIG. 1 is a block diagram of a communication system 100 that includes amobile station 102 that communicates through a wireless communicationnetwork. Mobile station 102 preferably includes a visual display 112, akeyboard 114, and perhaps one or more auxiliary user interfaces (UI)116, each of which is coupled to a controller 106. Controller 106 isalso coupled to radio frequency (RF) transceiver circuitry 108 and anantenna 110.

Typically, controller 106 is embodied as a central processing unit (CPU)which runs operating system software in a memory component (not shown).Controller 106 will normally control overall operation of mobile station102, whereas signal-processing operations associated with communicationfunctions are typically performed in radio frequency (RF) transceivercircuitry 108. Controller 106 interfaces with device display 112 todisplay received information, stored information, user inputs, and thelike. Keyboard 114, which may be a telephone type keypad or fullalphanumeric keyboard, is normally provided for entering data forstorage in mobile station 102, information for transmission to network,a telephone number to place a telephone call, commands to be executed onmobile station 102, and possibly other or different user inputs.

Mobile station 102 sends communication signals to and receivescommunication signals from the wireless network over a wireless link viaantenna 110. RF transceiver circuitry 108 performs functions similar tothose of a base station and a base station controller (BSC) (not shown),including for example modulation/demodulation and possiblyencoding/decoding and encryption/decryption. It is also contemplatedthat RF transceiver circuitry 108 may perform certain functions inaddition to those performed by a BSC. It will be apparent to thoseskilled in art that RF transceiver circuitry 108 will be adapted toparticular wireless network or networks in which mobile station 102 isintended to operate.

Mobile station 102 includes a battery interface (IF) 134 for receivingone or more rechargeable batteries 132. Battery 132 provides electricalpower to electrical circuitry in mobile station 102, and battery IF 134provides for a mechanical and electrical connection for battery 132.Battery IF 134 is coupled to a regulator 136 which regulates power tothe device. When mobile station 102 is fully operational, an RFtransmitter of RF transceiver circuitry 108 is typically keyed or turnedon only when it is sending to network, and is otherwise turned off toconserve resources. Similarly, an RF receiver of RF transceivercircuitry 108 is typically periodically turned off to conserve poweruntil it is needed to receive signals or information (if at all) duringdesignated time periods.

Mobile station 102 operates using a Subscriber Identity Module (SIM) 140which is connected to or inserted in mobile station 102 at a SIMinterface (IF) 142. SIM 140 is one type of a conventional “smart card”used to identify an end user (or subscriber) of mobile station 102 andto personalize the device, among other things. Without SIM 140, themobile station terminal is not fully operational for communicationthrough the wireless network. By inserting SIM 140 into mobile station102, an end user can have access to any and all of his/her subscribedservices. SIM 140 generally includes a processor and memory for storinginformation. Since SIM 140 is coupled to SIM IF 142, it is coupled tocontroller 106 through communication lines 144. In order to identify thesubscriber, SIM 140 contains some user parameters such as anInternational Mobile Subscriber Identity (IMSI). An advantage of usingSIM 140 is that end users are not necessarily bound by any singlephysical mobile station. SIM 140 may store additional user informationfor the mobile station as well, including datebook (or calendar)information and recent call information.

Mobile station 102 may consist of a single unit, such as a datacommunication device, a multiple-function communication device with dataand voice communication capabilities, a personal digital assistant (PDA)enabled for wireless communication, or a computer incorporating aninternal modem. Alternatively, mobile station 102 may be amultiple-module unit comprising a plurality of separate components,including but in no way limited to a computer or other device connectedto a wireless modem. In particular, for example, in the mobile stationblock diagram of FIG. 1, RF transceiver circuitry 108 and antenna 110may be implemented as a radio modem unit that may be inserted into aport on a laptop computer. In this case, the laptop computer wouldinclude display 112, keyboard 114, one or more auxiliary UIs 116, andcontroller 106 embodied as the computer's CPU. The display may be aliquid crystal display (LCD), such as an LCD with colour filters or afield sequential LCD. It is also contemplated that a computer or otherequipment not normally capable of wireless communication may be adaptedto connect to and effectively assume control of RF transceiver circuitry108 and antenna 110 of a single-unit device such as one of thosedescribed above. Such a mobile station 102 may have a more particularimplementation as described later in relation to mobile station 202 ofFIG. 2.

FIG. 2 is a detailed block diagram of a preferred mobile station 202.Mobile station 202 is preferably a two-way communication device havingat least voice and advanced data communication capabilities, includingthe capability to communicate with other computer systems. Depending onthe functionality provided by mobile station 202, it may be referred toas a data messaging device, a two-way pager, a cellular telephone withdata messaging capabilities, a wireless Internet appliance, or a datacommunication device (with or without telephony capabilities). Mobilestation 202 may communicate with any one of a plurality of fixedtransceiver stations 200 within its geographic coverage area.

Mobile station 202 will normally incorporate a communication subsystem211, which includes a receiver, a transmitter, and associatedcomponents, such as one or more (preferably embedded or internal)antenna elements and, local oscillators (LOs), and a processing modulesuch as a digital signal processor (DSP) (all not shown). Communicationsubsystem 211 is analogous to RF transceiver circuitry 108 and antenna110 shown in FIG. 1. As will be apparent to those skilled in field ofcommunications, particular design of communication subsystem 211 dependson the communication network in which mobile station 202 is intended tooperate.

Network access is associated with a subscriber or user of mobile station202 and therefore mobile station 202 requires a Subscriber IdentityModule or “SIM” card 262 to be inserted in a SIM IF 264 in order tooperate in the network. SIM 262 includes those features described inrelation to FIG. 1. Mobile station 202 is a battery-powered device so italso includes a battery IF 254 for receiving one or more rechargeablebatteries 256. Such a battery 256 provides electrical power to most ifnot all electrical circuitry in mobile station 202, and battery IF 254provides for a mechanical and electrical connection for it. The batteryIF 254 is coupled to a regulator (not shown) which provides power V+ toall of the circuitry.

Mobile station 202 includes a processor 238 (which is one implementationof controller 106 of FIG. 1) which controls overall operation of mobilestation 202. Communication functions, including at least data and voicecommunications, are performed through communication subsystem 211.Processor 238 may be an integrated circuit such as a microprocessor, aprocessing core on an integrated circuit, a processor of a system on achip, or the like. Processor 238 also interacts with additional devicesubsystems such as a display 222, a flash memory 224, a random accessmemory (RAM) 226, auxiliary input/output (I/O) subsystems 228, a serialport 230, a keyboard 232, a speaker 234, a microphone 236, a short-rangecommunications subsystem 240, and any other device subsystems generallydesignated at 242. Some of the subsystems shown in FIG. 2 performcommunication-related functions, whereas other subsystems may provide“resident” or on-device functions. Notably, some subsystems, such askeyboard 232 and display 222, for example, may be used for bothcommunication-related functions, such as entering a text message fortransmission over a communication network, and device-resident functionssuch as a calculator or task list. Operating system software used byprocessor 238 is preferably stored in a persistent store such as flashmemory 224, which may alternatively be a read-only memory (ROM) orsimilar storage element (not shown). Those skilled in the art willappreciate that the operating system, specific device applications, orparts thereof, may be temporarily loaded into a volatile store such asRAM 226.

Processor 238, in addition to its operating system functions, preferablyenables execution of software applications on mobile station 202. Apredetermined set of applications which control basic device operations,including at least data and voice communication applications, willnormally be installed on mobile station 202 during its manufacture. Apreferred application that may be loaded onto mobile station 202 may bea personal information manager (PIM) application having the ability toorganize and manage data items relating to the user such as, but notlimited to, instant messaging (IM), e-mail, calendar events, voicemails, appointments, and task items. Naturally, one or more memorystores are available on mobile station 202 and SIM 262 to facilitatestorage of PIM data items and other information.

The PIM application preferably has the ability to send and receive dataitems via the wireless network. In a preferred embodiment, PIM dataitems are seamlessly integrated, synchronized, and updated via thewireless network, with the mobile station user's corresponding dataitems stored and/or associated with a host computer system therebycreating a mirrored host computer on mobile station 202 with respect tosuch items. This is especially advantageous where the host computersystem is the mobile station user's office computer system. Additionalapplications may also be loaded onto mobile station 202 through network200, an auxiliary I/O subsystem 228, serial port 230, short-rangecommunications subsystem 240, or any other suitable subsystem 242, andinstalled by a user in RAM 226 or preferably a non-volatile store (notshown) for execution by processor 238. Such flexibility in applicationinstallation increases the functionality of mobile station 202 and mayprovide enhanced on-device functions, communication-related functions,or both. For example, secure communication applications may enableelectronic commerce functions and other such financial transactions tobe performed using mobile station 202.

In a data communication mode, a received signal such as a text message,an e-mail message, or web page download will be processed bycommunication subsystem 211 and input to processor 238. Processor 238will preferably further process the signal for output to display 222, toauxiliary I/O device 228 or both as described further herein below withreference to FIGS. 3 and 4. A user of mobile station 202 may alsocompose data items, such as e-mail messages, for example, using keyboard232 in conjunction with display 222 and possibly auxiliary I/O device228. Keyboard 232 is preferably a complete alphanumeric keyboard orkeypad and/or telephone-type keypad. These composed items may betransmitted over a communication network through communication subsystem211.

For voice communications, the overall operation of mobile station 202 issubstantially similar, except that the received signals would be outputto speaker 234 and signals for transmission would be generated bymicrophone 236. Alternative voice or audio I/O subsystems, such as avoice message recording subsystem, may also be implemented on mobilestation 202. Although voice or audio signal output is preferablyaccomplished primarily through speaker 234, display 222 may also be usedto provide an indication of the identity of a calling party, duration ofa voice call, or other voice call related information, as some examples.

Serial port 230 in FIG. 2 is normally implemented in a personal digitalassistant (PDA)-type communication device for which synchronization witha user's desktop computer is a desirable, albeit optional, component.Serial port 230 enables a user to set preferences through an externaldevice or software application and extends the capabilities of mobilestation 202 by providing for information or software downloads to mobilestation 202 other than through a wireless communication network. Thealternate download path may, for example, be used to load an encryptionkey onto mobile station 202 through a direct and thus reliable andtrusted connection to thereby provide secure device communication.

Short-range communications subsystem 240 of FIG. 2 is an additionaloptional component which provides for communication between mobilestation 202 and different systems or devices, which need not necessarilybe similar devices. For example, subsystem 240 may include an infrareddevice and associated circuits and components, or a Bluetooth™communication module to provide for communication with similarly-enabledsystems and devices. Bluetooth™ is a registered trademark of BluetoothSIG, Inc.

In accordance with an embodiment of the invention, mobile station 202 isa multi-tasking handheld wireless communications device configured forsending and receiving data items and for making and receiving voicecalls. To provide a user-friendly environment to control the operationof mobile station 202, an operating system resident on station 202 (notshown) provides a graphical user interface (GUI) having a main screenand a plurality of sub-screens navigable from the main screen.

A preferred embodiment of the liquid crystal display cell 222 is shownin greater detail in FIG. 3 in which a light source formed from multipleLEDs 322, 324, 326 is used as a backlight. The LCD may be a passivematrix or active matrix device. Preferably, the LCD is a full colourdevice. More preferably, the LCD is a field sequential LCD (FS LCD)—anLCD that obviates a need for colour filters. Alternatively, the LCD maycontain colour filters in which case a white light source may be used.LCD controller 316 provides a voltage to the common electrode(s) 308 andthe active elements 310 of the active matrix. The active elements arepreferably thin film transistors. The common electrode(s) 308 and activeelements of the LCD are supported on substrates 306 and 312,respectively. The LCD preferably contains a brightness enhancing film orlayer 340 to optimize the distribution of light for a viewer. As thepreferred liquid crystal material is super twisted nematic, polarizers302 and 314 are used. The LCD controller 316 sets the pixel grey scaleof the LCD. An optional processor 318 may coordinate synchronization ofthe LCD controller 316 with the light source controller 320. Preferably,the LCD controller 316 and the processor 318 are integrated into asingle device 317, which may simply be referred to as an LCD controllerthat has the capability of controlling a light source controller 320.The light source may be implemented by using red, green, and blue LEDs322, 324, 326. In a specific embodiment, four green, four red, and twoblue LEDs are used to provide full colour and/or black and whitedisplay. Other combinations of LEDs are contemplated by the presentinvention. The LED controller 320 may sequence the three colours or maysimultaneously energize LEDs of all the colours and terminate power tothe LEDs simultaneously. The light guide 328 may have a tapered blockconstruction and may have approximately a trapezoidal form to moreevenly distribute the light into the LCD. The light guide may also haveuneven areas 332 that scatter the light so as to avoid shadowing effectsin the LCD image. Although uneven area 332 is shown to project inward tothe surface of the light guide 328, the uneven areas may be arrangeddifferently so long as the arrangement effectively scatters the lightfrom the LEDs 322, 324, 326. The uneven areas may be abraded, molded,corrugated, chemically etched, or the like. Preferably, to maximize theutilization of light, the LEDs 322, 324, 326 and the light guide 328 arepartially enclosed by a reflector such that the only opening is fullybounded by the light transmissive area of the LCD.

FIG. 4 illustrates an exemplary embodiment of a light guide and acompensation sheet. The left image of FIG. 4 shows hot spots as seenfrom the LCD side of the light guide. The hot spot image may be captureddirectly from the light guide when the light guide is illuminated fromits light source or may be captured through a filter. A mask is formedfrom the captured image in a pattern that mirrors the pattern of lightfrom an illuminated light guide. The mask, shown in the centre image ofFIG. 4, may be a sheet or film with apertures. The hot spot filter maybe a transparent sheeting with printing or coating in select areascorresponding to the hot spots. The printing or coating may be lightabsorptive or partially light reflective. Light absorbing pigments mayoffer lower cost in making. Partially light reflective (and partiallylight transmissive) coatings aid to recycle light and so reduce powerconsumption since less power needs to be expended for illumination. Thedegree of reflectivity/transmissivity of a hot spot compensating regionon a hot spot filter may be adjusted according to the particularapplication. Alternative to a transparent sheet with printing orcoating, a bitmap correction filter may be applied for a softwaresolution to the LCD image such as when a shutter (e.g., another LCD) isused to establish lowered transmissivity regions over the hot spots. Theend result of the hot spot compensation layer or shutter is a uniformplanar light to the LCD.

FIG. 5 illustrates an exemplary embodiment of an optical system thatcaptures the light distribution from a light guide 506 when illuminatedby a light source 508. The light source may include one or more lightemitting diodes. For a full colour liquid crystal display, an image maybe captured for each colour of light. Capturing an image for each colourof light allows for a more accurate hot spot filtering pattern toaccount for physical offsets in location for the light emitting diodesof each colour. For example, because a red LED is not located where agreen LED is, the light output pattern from a light guide illuminated bythe red LED is not likely to be identical to the light output patternfrom the light guide illuminated by the green LED. Furthermore, a lightguide image may be obtained from each individual LED of the backlight.This is especially useful where more than one LED is used per colour.The light output pattern from the light guide 506 may be opticallyprocessed through an optical system 504 that transfers to appropriatelyscaled imaged to a receiving sensor array with a camera 502. In anembodiment, the optical system 504 may include one or more lenses. Thecamera 502 may, for example, include a charge-coupled device (CCD).

FIG. 6 illustrates an embodiment of a method for creating a hot spotfilter for the light guide. The light guide is illuminated to provide alight output pattern by an individual LED (e.g., white, red, green, orblue) or a group of LEDs representing a colour in step 602. The lightoutput pattern may be optically processed. This light output pattern iscaptured as an image by a light-capturing device, such as a camera 604.The captured image may be used to make a mask 606. This mask, in turn,is used to form the hot spot filter 608. For example, the mask maycontain apertures shaped and sized to conform to the hot spots. The maskmay be laid over or upon a substrate, such as a plastic or glass film,and a coating or printing process deposits reflective and/or absorptivematerial onto the substrate through these apertures. After furtherprocessing (e.g., heating, drying, ultraviolet or infrared curing), thesubstrate is ready for disposition over the light guide in the finishedproduct. The image capturing process may be performed from an actualhandheld wireless device that is opened to expose the light guide. Insuch case, the camera may be disposed at a distance corresponding tothat of the LCD's location within the finished product. This imagecapturing process may be performed once per batch or at a designatedsampling rate to account for component devices from batch to batch.

FIG. 7 illustrates another method for creating a hot spot filter. InFIG. 7, a colour grey scale compensation map is created for each colourof light illuminating the LCD. First, one of the colours is selected 702in which light of that colour illuminates the light guide 706. The imagefrom the light guide for that colour is captured 708. The captured imageis processed to form a compensation map 710 to eliminate hot spotsthrough use of the hot spot filter. In this case, the hot spot filtermay be a film or layer (350 of FIG. 3) upon which printing or coating ofthe hot spot areas is performed, may be such printing or coatingdirectly upon the light guide, may be another liquid crystal displaythat is able to alter the pixel by pixel grey scale to compensate forany hot spots, or may be the primary LCD (defined by substrates 312 and306 in FIG. 3) in which certain pixels are given an added darkened greyscale value for compensation. The grey scale compensation map for thecolour is stored in a suitable memory (e.g., the flash memory). Themethod determines if another colour of LED is to be processed 714, 704.In this way, distinct image patterns may be obtained for each colour oflight generated by the light source.

The above-described embodiments of the present application are intendedto be examples only. Those of skill in the art may effect alterations,modifications and variations to the particular embodiments withoutdeparting from the scope of the application. Instead of a liquid crystaldisplay, a keypad or keyboard may be illuminated using the disclosedtechniques. The invention described herein in the recited claims intendsto cover and embrace all suitable changes in technology.

1. A light outputting arrangement, comprising: a light guide; a lightsource for illuminating the light guide wherein any bright spots of anoutput light pattern from the light guide are compensated to effect auniform output light pattern; a layer in proximity to the light guide toprovide the compensation to effect a uniform output light pattern, thelayer having a first area of a first transmissivity and a second area ofa second transmissivity.
 2. The light outputting arrangement of claim 1,wherein the area of a first transmissivity includes grey printing. 3.The light outputting arrangement of claim 1, further comprising a liquidcrystal display that receives the compensated light output pattern fromthe light guide.
 4. The light outputting arrangement of claim 1, furthercomprising a keypad that receives the compensated light output patternfrom the light guide.
 5. The light outputting arrangement of claim 1,further comprising: a liquid crystal display (LCD); a storedrepresentation of a first compensation pattern to block light from thefirst area relative to light from the second area; a module toincorporate the stored representation into an image to be displayed onthe LCD; and a module to generate the image with the storedrepresentation for display on the LCD, wherein when the firstcompensation pattern is generated on the LCD, the first compensationpattern aligns with the first and second areas to reduce the intensityof light passing through the first compensation pattern from the firstarea relative to the intensity of light passing through the firstcompensation pattern from the second area.
 6. The light outputtingarrangement of claim 5, wherein the LCD is a field sequential LCD. 7.The light outputting arrangement of claim 6, further comprising a layerbetween the light guide and the LCD, the layer providing a secondcompensation pattern to further reduce the intensity of light emittedfrom the first portion relative to the intensity of light emitted fromthe second portion.
 8. The light outputting arrangement of claim 7,wherein the layer includes a transparent substrate.
 9. The lightoutputting arrangement of claim 8, wherein the transparent substrate hasat least one area that transmits light different from another area ofthe transparent substrate.